The present invention relates to methods and apparatus in which liquid is vaporized and pressurized in an enclosed porous member, and relates particularly to methods and apparatus for vaporizing liquid fuels to produce a combustible mixture under pressure. Combustion apparatus employing a vaporization/pressurization module and combustion methods of the present invention are especially suitable for use as light and heat sources for stoves, burners, lamps, appliances, thermal to electric conversion systems and the like.
Conventional boilers add heat to a reservoir or inflow of liquid to convert the liquid to vapor. To sustain the inflow of liquid in a pressurized boiler system, the liquid must be supplied under at least as much pressure as that of the outgoing vapor. In a typical industrial boiler, the liquid is pumped into the boiler according to the desired vapor pressure. A throttle controls the flow of vapor from the boiler and, correspondingly, the vapor pressure within the boiler. Feed pumps supply water to the boiler according to the vapor pressure to maintain a constant liquid level in the boiler. If the vapor pressure is increased by reducing flow through the throttle, then the pumping pressure is decreased to maintain the level of liquid hi the boiler. Usually, the throttle is operatively coupled to the feed pump(s) so that the pumping pressure is automatically adjusted according to the flow through the throttle and, correspondingly, the vapor pressure in the boiler. This mechanism of automatically controlling the performance of the feed pumps is commonly referred to as a servomechanism.
In most liquid fuel vaporization applications, liquid fuel is vaporized, then mixed with air or an oxygen-containing gas, and the vaporized fuel/gas mixture is ignited and burned. The liquid fuel is generally supplied under pressure, and vaporized by mechanical means or heated to vaporization temperatures using an external energy source.
Portable burners and light sources that utilize liquid fuels generate liquid fuel vapor, which is then mixed with air and combusted. Combustion devices that burn fuels that are liquids at atmospheric temperatures and pressures, such as gasoline, diesel fuel and kerosene, generally require the liquid fuel to be pressurized by a pump or other device to provide vaporized fuel under pressure. Fuels such as propane and butane, which are gases at atmospheric pressures but liquids at elevated pressures, can also be used in portable burners and light sources. Storage of these fuels in a liquid form necessitates the use of pressurized fuel canisters that are inconvenient to use and transport, are frequently heavy, may he explosion hazards, and require valves which are prone to leaking.
The fuel boiler of propane and butane burners is the reservoir or storage tank itself, from which the gases are released under pressure as vapor. When vapor is withdrawn from the fuel reservoir, the pressurized reservoir acts as a boiler, and draws the required heat of vaporization from ambient air outside the tank. These systems have many disadvantages. The vapor pressure of propane inconveniently depends upon ambient temperature, and the vapor pressure is generally higher than that needed for satisfactory combustion in a burner. While butane fuel has an advantageous lower vapor pressure than propane, burners using butane have difficulty producing sufficient vapor pressure at low ambient temperatures. Burners using a mixture of propane and butane fuel provided under pressure in disposable canisters have also been developed. This fuel mixture performs well at high altitudes, but still does not perform well at low ambient temperatures.
A needle valve can be used to control propane vapor at tank pressure to regulate the fuel flow, and thus the heat output, of a burner. Burner control using a needle valve tends to be delicate and sensitive to ambient temperatures. Alternatively, a pressure regulator can be used to generate a constant and less hazardous pressure of propane that is independent of tank temperature. Propane pressure regulators are commonly used in outdoor grills, appliances for recreational vehicles and boats, and domestic propane installations. Unfortunately, regulators are bulky and are seldom practical for application to small scale portable burner devices.
Despite considerable development efforts and the high market demand for burners for use in stoves, lamps and the like, that operate safely and reliably under a wide variety of ambient temperature, pressure and weather conditions, commercially available combustion devices are generally unsatisfactory.
Wicking systems that use capillary action to convey and vaporize liquid fuels at atmosphenic pressure are known for use in liquid fuel burners. U.S. Pat. No. 3,262,290, for example, discloses a liquid fuel burner in which a wick stone is fastened in a fuel storage container and feeds liquid fuel from the fuel reservoir to the burner. In this system, liquid fuel is provided to the wick stone by an absorbent textile wick, and the wick stone is biased against a burner wick.
U.S. Pat. No. 4,365,952 discloses a liquid fuel burner in which liquid fuel is drawn up from a reservoir by a porous member having a fuel receiving section and a fuel evaporation section. Liquid fuel is supplied by capillary action at a rate matching the rate of evaporation of the fuel. Air is supplied to the fuel evaporation section, and liquid fuel is evaporated from the surface at a rate corresponding to the rate of air supply. The gaseous fuel and air is mixed and jetted from a flame section to a burning section. An externally powered heater maintains the porous member of the fuel evaporation section substantially at a constant temperature irrespective of the rate of evaporation of the liquid fuel.
U.S. Pat. No. 4,421,477 discloses a combustion wick comprising a fuel absorption and a fuel gasifying portion designed to reduce the formation and deposition of tar-like substances in the wick. The wick comprises silica-alumina ceramic fibers molded with an organic binder, with part of the wick provided with a coating of an inorganic pigment, silicic anhydride and a surface active agent. The wick preferably has a capillary bore size of about 1 to 50 microns, with smaller pore size wicks being less prone to accumulation of tar-like substances on the inside.
U.S. Pat. No. 4,465,458 discloses a liquid fuel combustion system in which the liquid fuel is drawn into a porous fiber material or fabric, which is intimately contacted by an externally powered heat generating member to evaporate and vaporize the liquid fuel. Air is introduced to promote vaporization of the liquid fuel and provide an admixed liquid/fuel mixture for burning. Combustion is variable by adjusting the heat input and the air supply.
U.S. Pat. No. 4,318,689 discloses a burner system in which liquid fuel is pumped into a cylindrical chamber having a porous side wall. As a result of the pressure differential, the liquid fuel penetrates the porous wall to form a film on the external surface of the porous chamber wall. Preheated combustion air entrains and vaporizes the liquid fuel film formed on the external wall of the chambers and circulates the fuel/air mixture to a combustion chamber. A portion of the hot exhaust or combustion gases may be returned for countercurrent heat exchange to preheat the combustion air.
Although the prior art discloses numerous types of liquid fuel combustion systems, most liquid fuel vaporizers require the application of energy from all external source, such as heat energy, pressure for pressurizing the liquid fuel and/or vapor, or a blower for jetting an air stream to entrain the vaporized fuel for burning. Prior art liquid fuel combustion systems generally provide vaporization of liquid fuels at atmospheric pressures or, if a pressurized vapor stream is desired, either require the fuel supply to be pressurized or pressurize the vapor by external means. Many of the systems are complex and are not suitable for liquid fuel combustion apparatus that are robust, portable or that are suitable for small scale heating or lighting applications.
It is, therefore, an object of the present invention to provide an apparatus for vaporization and pressurization of liquids, including liquid fuels, within a vaporization/pressurization module having a porous member.
It is another object of the present invention to provide a vaporization/pressurization module that produces a pressurized vapor jet from liquid such as liquid fuel supplied at ambient pressures without requiring the use of pumps or other mechanical means.
It is yet another object of the present invention to provide a vaporization/pressurization module that produces a vapor jet at substantially constant pressures and at a substantially steady flow rate.
It is still another object of the present invention to provide a combustion apparatus employing a vaporization/pressurization module to vaporize liquid fuels, and to produce a pressurized fuel vapor jet.
It is yet another object of the present invention to provide a liquid fuel combustion apparatus that, following ignition, operates in a closed-loop feedback, steady state system that does not require energy input from an external source.
It is still another object of the present invention to provide a liquid fuel combustion apparatus which does not require priming and in which combustion and steady state operation can be conveniently initiated by application of heat from a match or lighter.
It is yet another object of the present invention to provide a liquid fuel combustion apparatus that can operate using any one of two or more different types of liquid fuel.
It is still another object of the present invention to provide a simplified combustion apparatus that generates heat and light by combustion of vaporized, pressurized liquid fuel that can be conveniently provided in a lightweight, portable and/or miniaturized form,
The liquid vaporization and pressurization apparatus of the present invention utilizes a vaporization/pressurization module employing a porous member having a low thermal conductivity and a substantially uniform, small pore size. The porous member has a liquid feed surface in proximity to a liquid feed system and a vaporization zone in proximity to a heat source. Liquid feed is introduced to the porous member at the liquid feed surface and is heated, vaporized and pressurized within and/or on a surface of the porous member. Egress of vapor to a location remote from the porous member is substantially constrained or is substantially constrainable by means of a substantially vapor impermeable barrier provided in proximity to surfaces of the porous member other than the liquid feed surface. The substantially vvapor impermeable barrier facilitates accumulation and pressurization of the vapor, which is released from the vaporization/pressurization module as a pressurized vapor jet from one or more restricted passage(s) formed in the substantially vapor impermeable barrier.
The barrier is referred to herein as xe2x80x9csubstantiallyxe2x80x9d vapor impermeablle because it is vapor impermeable except in predetermined locations where egress of one or more pressurized vapor jet(s) is permitted. The substantially vapor impermeable barrier facilitates pressurization of vapor within the porous member and the enclosed space formed by the barrier, and promotes generation of one or more vapor jet(s) at a pressure greater than that of the liquid feed which is generally provided at atmospheric pressure. According to preferred embodiments, egress of vapor is limited by a substantially vapor impermeable barrier having one or more restricted passage(s) permitting egress of pressurized vapor, the passage(s) constituting less than about 5%, more preferably less than 2%, and most preferably less than about 0.5%, of the surface area of the substantially impermeable barrier.
The vaporization/pressurization module of the present invention may be provided as an independent unit for a variety of applications. The vaporization/pressurization module comprises a porous member, a heat source and a substantially vapor impermeable barrier. A liquid feed system provides liquid to the vaporization/pressurization module. Liquid is generally provided at ambient temperatures and pressures to the liquid feed surface of the porous member and is drawn into the porous member and conveyed to a vaporization zone within and/or on a surface of the porous member by capillary forces. During operation, the heat source is used to establish and maintain a thermal gradient within the porous member between the liquid feed surface and the vaporization zone. Liquid drawn into the porous member is heated as it traverses the porous member until it reaches its vaporization temperature in the vaporization zone. Vapor pressure within the vaporization/pressurization module accumulates as liquid is vaporized, and is maintained as a consequence of the substantially vapor impermeable barrier. One or more pressurized vapor jet(s) exit the substantially vapor impermeable barrier only at one or more restricted passage(s).
For liquid fuel combustion applications, a burner assembly is provided in combination with the vaporization/pressurization module and liquid feed system to facilitate mixing, of fuel vapors to form a combustible mixture and to provide a combustion zone. A liquid fuel feed system, such as a gravity-fed system or a capillary feed system employing a porous capillary feed wick or capillary tube(s), conveys liquid fuel from a fuel reservoir to the liquid feed surface of the porous member, which is generally at the xe2x80x9cbottomxe2x80x9d of the porous member. The liquid fuel feed system may be provided as an integral component of the porous member for certain applications. The heat source may be provided as a heating element using an extenial power source, or a portion of the heat generated by combustion may be retutned to provide the heat required for vaporization. A substantially vapor impermeable barrier may be provided, for example, in the form of: (i) a vapor impermeable shroud positioned in proximity to porous member surfaces adjacent the liquid feed surface; in combination with (ii) a substantially vapor impermeable plate having one or more restricted passage(s) positioned in proximity to a porous member surface opposite the liquid feed surface.
According to especially preferred embodiments, the vapor impermeable shroud has a generally low thermal conductivity, while the substantially vapor impermeable plate has a generally high thermal conductivity. When the porous member is provided as a generally cylindrical or rectangular member, the liquid feed surface is generally the xe2x80x9cbottomxe2x80x9d surface, a vapor impermeable shroud is positioned in proximity to the porous member sidewalls, and a substantially vapor impermeable plate is positioned in proximity to the porous member xe2x80x9ctopxe2x80x9d surface. The heat source may be provided at or near the xe2x80x9ctopxe2x80x9d of the porous member, for example, as a thermally conductive element deriving heat from a source internal or external to the combustion apparatus. When this arrangement is employed, the vaporization zone of the porous member is in proximity to and generally xe2x80x9cbelowxe2x80x9d the heat source. One or more vapor permeable passage(s) are preferably provided in the substantially vapor impermeable plate to permit egress of one or more fuel vapor jet(s) under pressure. Pressurized fuel vapor jet(s) entrain air or another gas or gas mixture to produce a combustible fuel/gas mixture. The combustible fuel/gas mixture may be ignited and burned continuously or intermittently in a combustion zone of the burner assembly.
Certain embodiments of combustion apparatus of the present invention do not require priming or a discrete starter mechanism to initiate liquid fuel vaporization, pressurization and combustion. In one preferred combustion apparatus, heat applied briefly to the burner assembly by a match or lighter is conducted to the porous member and is sufficient to initiate liquid fuel vaporization on or within the porous member, leading to pressurization of the fuel vapor in the vaporization/pressurization module and combustion of the resulting combustible mixture. Once combustion is initiated, the heat for fuel vaporization and pressurization is preferably derived by returning a portion of the heat generated by combustion to the porous member, for example, through conductive elements forming a part of the burner in thermal communication with a hot seat having a high thermal conductivity. The hot seat is preferably located in proximity to and in thermal communication with both the porous member and the burner to transfer the heat energy necessary for fuel vaporization and pressurization from the burner to the porous member. According to preferred embodiments, a steady state condition can be achieved and maintained wherein liquid fuel provided to the liquid feed surface of the porous member at substantially ambient pressures and temperatures is heated and pressurized within the vaporization/pressurization module using a portion of the heat generated in the burner to produce one or more pressurized vapor jet(s), which in turn are used for combustion.
Vaporization/pressurization modules and liquid feed systems of the present invention may be scaled to provide a range of pressurized vapor outputs. For liquid fuel applications, vaporization/pressurization modules may also be used with controllable, variable output combustion apparatus. The combustion output may be varied in numerous ways and is most conveniently varied by adjusting the vaporized, pressurized fuel stream(s) exiting from the module. Adjustment of the vaporized, pressurized fuel stream may be accomplished, for example, by adjusting the amount of heat supplied to the module, by adjusting the flow of liquid fuel to the liquid feed surface of the porous member, or by limiting or adjusting the egress of vaporized fuel from the module. The flow of liquid fuel to the porous member may be regulated by restricting capillary flow through the porous member or, where all assembly of multiple individual modules is used, by removing a selected number of them from the liquid. The flow of pressurized vapor from the module may be regulated by providing a valve or a throttle, or other mechanical means. The quantity of heat supplied to the porous member may be varied, for example, by adjusting the power provided an electrical resistive heating element or by modulating the amount of heat returned to the vaporization/pressurization module from combustion.
Combustion apparatus may incorporate a plurality of individual vaporization/pressurization modules and/or an array of burners, each burner associated with one or more vaporization/pressurization modules, in applications requiring a higher heat or light output than a single module or burner can provide. In addition, modules and/or burners having different capacities may be arrayed together for use separately or in combination.
The vaporization/pressurization module liquid feed system and combustion apparatus may be adapted for use in applications requiring a heat or light source, and are especially suitable for use in applications in which a portable heat and/or light source is required. Such combustion apparatus may be used with a variety of liquid fuels, including fuels such as gasoline, white gas, diesel fuel, kerosene, JP8, alcohols such as ethanol and isopropanol, biodiesel, and combinations of liquid fuels. Vaporization/pressurization modules, liquid feed system, and combustion apparatus of the present invention may be optimized for use with a particular liquid fuel source, or a single module feed system and combustion apparatus may be designed for use with multiple liquid fuels. The system is thus highly versatile and may take advantage of readily available fuels. The vaporization/pressurization module of the present invention may be used in connection with or used to retrofit any type of apparatus that requires the formation of a pressurized vapor jet from a liquid.
Combustion apparatus components other than the burner, the heat source, and the thermal path between the two remain cool to the touch during operation, and the liquid fuel need not be pressurized to provide a substantially continuous vaporized fuel jet during operation. The combustion apparatus of the present invention thus incorporates many safety features not available in other types of combustion apparatus. Moreover, combustion apparatus of the present invention may be miniaturized and constructed from lightweight materials. Simple embodiments of the combustion apparatus employing a vaporization/pressurization module, with or without a separate liquid feed system, may be designed to have few components, and no moving components. Such apparatus may be produced at a low cost and demonstrate improved reliability. They burn efficiently and xe2x80x9cclean,xe2x80x9d and are not prone to clogging as a result of oxidation or pyrolosis of the liquid fuel.
Combustion apparatus incorporating vaporization/pressurization modules and liquid feed systems of the present invention are especially suitable for use as portable heaters, stoves and lamps for indoor, outdoor and/or marine applications, as well as power sources for use in a variety of devices, including absorption refrigerators and other appliances, and thermal to electric conversion systems, such as thermophotovoltaic systems, thermoelectric thermopiles, and alkali metal thermal to electric conversion (AMTEC) systems. Applications including outdoor, camping and marine stoves, portable or installed heaters, lamps for indoor or outdoor use, including mantle lamps, torches, xe2x80x9ccanned heatxe2x80x9d for keeping food or other items warm, xe2x80x9ccanned lightxe2x80x9d as a replacement or supplement to candles or other light sources, and emergency heat and light xe2x80x9csticksxe2x80x9d are just a few of the many applications for such combustion apparatus. Exemplary non-combustion applications of vaporization/pressurization modules of the present invention include steam generation apparatus and other types of apparatus for providing liquids in a vaporized, aerosol or atomized form.